Naked pecs? Add muscle for striking effect (5)
I believe the answer is:
eclat
'striking effect' is the definition.
(I've seen this before)
'naked pecs? add muscle' is the wordplay.
'naked' means to look at the middle letters.
'add' means one lot of letters go next to another.
'muscle' becomes 'lat' (lat is a kind of muscle**).
The middle of 'pecs' is 'ec'.
'ec'+'lat'='ECLAT'
'for' acts as a link.
(Other definitions for eclat that I've seen before include "Splendour; renown" , "brilliant effort" , "Social distinction in France" , "Universal approval" , "Distinguished success" .)